1. Field of the Invention
This invention relates to IC fabricating processes, and more particularly relates to an alignment mark, and a method of getting a position reference for a wafer that is based on the alignment mark.
2. Description of the Related Art
A wafer for IC fabrication usually requires a position reference to be defined thereon before subjected to a process, such as a checking process like overlay checking process. A position reference is often got by forming two alignment marks in the pre-layer at different positions and locating the alignment marks as two reference points.
FIG. 1 illustrates a prior-art alignment mark 100 on a wafer 10, which serves as one of two reference points 12 on the wafer 10 and includes an L-shaped pattern 110 and its surrounding 120. The alignment mark 100 is located by detecting brightness difference between the L-shaped pattern 110 and its surrounding 120.
However, when there is an opaque film, such as an amorphous carbon film, in the stack of films, the contrast between the L-shaped pattern 110 and its surrounding 120 is insufficient, so that the alignment mark 100 is difficult to locate and the recipe cannot be setup. Although the contrast can be increased by using near-IR light in the detection, additional tools for manipulating the near-IR detection are required.